| Title | A Web Application for Geothermal Borefield Design |
|---|---|
| Authors | Davide ROLANDO, José ACUNA, Marco FOSSA |
| Year | 2015 |
| Conference | World Geothermal Congress |
| Keywords | ground coupled heat pumps, borehole heat exchangers, g-functions, thermal response test, web application |
| Abstract | The correct design of a borehole heat exchanger (BHE) system implies the accurate knowledge of borehole and ground thermal properties, the correct evaluation of building heating or cooling demands and the correct procedure to assess the final overall BHE length related to BHE configuration shape. A careful design is required to make profitable payback plans and provide expected long time performance. To describe and predict the heat transfer due to the interaction between a BHE field and the surrounding ground the assumptions of pure conduction and constant ground properties are widely adopted. Under those hypotheses some base solutions are available and mainly differ depending on whether the BHE is considered as an Infinite Line Source (ILS), an Infinite Cylindrical Source (ICS) or a Finite Line Source (FLS). Non-dimensional thermal response factor (g-functions) are also used. The evaluation of borehole effective thermal resistance and ground thermal conductivity can be assessed by means of a Thermal Response Test (TRT). The TRT is a worldwide adopted in-situ methodology based on heat rate injection (extraction) to (from) the ground through a fluid circulating in a pilot BHE. The measured fluid temperature evolution acquired during the test is analysed with numerical or analytical approaches often based on the above mentioned models. Once the BHE and ground thermal properties are known, the building energy demand profile is needed in order to evaluate the proper combination of overall BHE length and BHE location which fulfil the long term efficiency level. The Ashrae method allows the BHE system to be designed by considering three representative building heat loads, their respective thermal resistances based on ICS model and a Temperature Penalty variable which takes into account the long term BHE interaction effect. In this paper. the principle and description of a web-based toolkit for BHE design is presented. The web application intends to implement some of the main procedures universally adopted in BHE system design, including an improved Ashrae method and a g-function calculation tool for arbitrary borehole field geometries. This arbitrary shaped borefield design allows the designer to fit the peculiar needs of each application project. Current free or commercial programs let the user to design BHE system according to a priori determined set of available configuration in terms of borefield shape and borehole to borehole distance while real applications often require the borehole field geometry not to respect a regular matrix configuration. A section for TRT analysis allows the estimation of BHE and ground thermal properties. A section for analytical estimation of borehole effective thermal resistance based on literature review is also presented. The benefits of developing a web toolkit with shared knowledge which also modern mobile devices can employ is remarked towards the BHE technology potential spread. |